DS had this and, to put your mind at ease, he is perfectly fine and hasn't had any problems because of it. After my 20w ultrasound, they had me do a follow up ultrasound at about 32 weeks. It still had not resolved. So, when he was born, they did another ultrasound which showed that it was still there. They also did a test where they inject dye into the urethra and see if it refluxes back into the kidneys. They said that this can give them more information on why the area is dilated. DS was put on antibiotics and we went to see a urologist when he was about a week old. The urologist stopped the antibiotics right away because he said the dilation was mild, but that he still wanted to monitor it. So, we had ultrasounds every 6 months until DS was 18months at which point it had resolved.
I was not that worried when they told me at the 20w ultrasound. I was sure that it would resolve before birth like they said. When it didn't, I was a mess. I sat in my car and cried. So, I know it's scary, but I hope that my experience can help you feel better about it.